Questions about the earliest stages in the origin of life on Earth may never be fully answered because, as far as we know, there is no record of these ancient events. Many later events, however, are well documented in the fossil record. Careful study of fossils opens a window into the lives of organisms that existed long ago and provides information about the evolution of life over billions of years.
Unity and diversity of life:
As diverse as life is, there is also evidence of remarkable unity, especially at the molecular and cellular levels. DNA, the universal genetic language is common to organisms as different as bacteria and animals and among eukaryotes, unity is evident in many features of cell structure.